Lithium electrochemical cell activated with an electrolyte containing LiBOB and FEC additives

ABSTRACT

An electrochemical cell having a casing housing an electrode assembly of a separator residing between a lithium anode and a cathode comprising silver vanadium oxide and fluorinated carbon is described. The electrode assembly is activated with a nonaqueous electrolyte comprising a lithium salt dissolved in a solvent system of propylene carbonate mixed with 1,2-dimethoxyethane, lithium bis(oxalato)borate (LiBOB), and fluoroethylene carbonate (FEC). Preferably LiBOB is present in an amount ranging from about 0.005 wt. 5 to about 5 wt. %, and FEC is present in an amount ranging from about 0.01 wt. % to about 10 wt. %. This electrolyte formulation is more conductive than the conventional or prior art binary and ternary solvent system electrolytes while being chemically and electrochemically stable toward Li/SVO cells, Li-SVO/CF x  mixture cells, and Li-SVO/CF x  sandwich cathode primary electrochemical cells.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 62/992,342, filed on Mar. 20, 2020.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention generally relates to the conversion of chemical energy toelectrical energy. More particularly, the present invention relates toprimary lithium-containing electrochemical cells activated withnonaqueous electrolytes. An exemplary couple is a lithium anode pairedwith a silver vanadium oxide (SVO) cathode or with a SVO/carbonmonofluoride (CF_(x)) cathode. If the latter, SVO and CF_(x) arepreferably contacted to opposite sides of a current collector, forexample, in a SVO/current collector/CF_(x)/current collector/SVOsandwich-type configuration. In another embodiment, SVO and CF_(x) aremixed together and the mixture is contacted to the opposite sides of thecurrent collector.

In particular, the present invention relates to an electrolytecomprising a new solvent system for activating primary Li/SVO,lithium-SVO/CF_(x) mixture, and lithium-SVO/CF_(x) sandwich cathodeactive material couples. These types of electrochemical cells are usefulfor powering implantable medical devices, for example cardioverterdefibrillators (ICD) and cardiac resynchronization therapy devices(CRT-D), which require high rates of discharge, stable operation, andpredictable end-of-life.

2. Prior Art

The successful production of lithium electrochemical cells and theirwidespread application is largely dependent on the development of highlyconductive and stable nonaqueous organic electrolytes. A generalrequirement of a nonaqueous organic electrolyte is that it isreductively and oxidatively stable toward both typically used anodeactive materials, for example, lithium metal and lithiated carbon, andtypically used cathode active materials, for example, silver vanadiumoxide (SVO), fluorinated carbon (CF_(x)), copper silver vanadium oxide(CSVO), manganese oxide (MnO₂), cobalt oxide (CoO₂), and others. For ahigh rate lithium cell application, an activating electrolyte with highconductivity is especially significant. Conventional nonaqueous organicelectrolytes are composed of a lithium salt dissolved in an organicsolvent system of either a single solvent or mixed solvents. However, toachieve high electrolyte conductivity, a combination of two solvents,one with a high dielectric constant and one with a low viscosity, isgenerally used.

In that respect, preferred lithium salts include LiAsF₆, LiPF₆, LiBF₄,LiClO₄, LiSO₃CF₃, among others. Typically used solvents includepropylene carbonate (PC), ethylene carbonate (EC), y-butyrolactone(GBL), sulfolane, 1,2-dimethoxyethane (DME), dimethyl carbonate (DMC),tetrahydrofuran (THF), diisopropyl ether (DIPS) 1,3-dioxolane, andothers. Particularly stable and highly conductive electrolytes that havefound widespread use in cells powering implantable medical devices, forexample cardioverter defibrillators (ICD) and cardiac resynchronizationtherapy devices (CRT-D), have either 1.0M to 1.2M LiAsF₆ or 1.0M LiPF₆dissolved in a 50:50, by volume, mixture of PC:DME. A particularlycommon electrolyte for high rate Li/SVO defibrillator cells uses LiAsF₆as the preferred salt. Despite the success of 1.0M LiAsF₆/PC:DME=1:1 asan electrolyte, an improved electrolyte with higher conductivity andincreased stability is desired for activating high rate, high power, andhigh capacity lithium primary electrochemical cells.

It is interesting to note that the above-discussed electrolyte using aPC/DME solvent system does not provide maximum conductivity at itsone-to-one volume ratio. As shown in FIG. 1 , the maximum conductivityof 1.0M LiAsF₆ dissolved in PC/DME is at a volume ratio of 20:80. Theconductivity of DME of about 19.5 mohms/cm at 37° C. is about 12.4%higher than that of PC of about 17.3 mohms/cm at 37° C. Consequently,the benefit of using an electrolyte of 1.0M LiAsF₆ dissolved in asolvent system of PC:DME at a volume ratio that is less than 50:50, forexample at a ratio of 20:80, by volume, which has a higher conductivity,seems obvious for high rate lithium electrochemical cells.

However, an acceptable electrolyte must provide both high conductivityand high stability toward both the anode and cathode active materials.The high conductivity requirement is needed to significantly reduce orminimize internal resistance (IR) voltage drop during high current pulsedischarge. The high stability requirement is desired to minimizeimpedance build-up at the solid electrolyte interface (SEI) at the anodeand at the cathode. Therefore, high electrolyte conductivity does notnecessarily mean better cell performance or improved discharge capacity.Indeed, when electrolytes of 1.0M LiAsF₆/PC:DME=40:60 or 30:70, byvolume, are used to activate Li/SVO cells, the benefit of their highconductivity in a short term discharge test is completely cancelled bythe presence of undesirable voltage delay resulting from impedancebuild-up at the solid electrolyte interface (SEI) during high currentpulse discharge applications.

In that respect, it is believed that voltage delay in Li/SVO cells is atleast partially related to electrolyte instability caused by thedissolution of vanadium ions from the SVO cathode active material intothe electrolyte. The vanadium ions then re-deposit on the anode surfaceby reduction to produce a highly resistant surface film. The iondissolution process is catalyzed by the presence of DME, which is a verygood ligand molecule. As a linear ether, DME has a larger donationnumber (DN=20) than that of propylene carbonate having a DN=15.1. Thedonation number signifies the potential of a nucleophile molecule todonate an electron pair as described in the Lewis acid-base theory. Tominimize or even eliminate the voltage delay phenomenon, a lowerpercentage of DME in the electrolyte solvent mixture is desired.However, by reducing the percentage of DME having a relatively high DN,the electrolyte conductivity is also decreased. Therefore, theelectrolyte of 1.0M LiAsF₆/PC:DME=50:50, by volume, which is typicallyused to activate Li/SVO cells powering implantable medical devices,represents a compromise between maximizing the solvent system'sconductivity and contemporaneously minimizing the undesirable effect ofvanadium ions that have dissolved into the electrolyte.

Although the 1.0M LiAsF₆/PC:DME=50:50, by volume, electrolyte issatisfactory when activating Li/SVO, lithium-SVO/CF_(x) mixture, andlithium-SVO/current collector/CF_(x)/current collector/SVO sandwichcathode cells used in cardioverter defibrillator and cardiacresynchronization therapy applications, and the like, its stability isless than desirable as it slowly decomposes to form a relatively highlyresistive surface film on the opposite polarity electrodes at certaindischarge values. This is signified by the voltage delay phenomena. Forlonger-term cell storage or usage, the voltage delay phenomenon becomesincreasingly more obvious and severe. Therefore, in order to fullyrealize the improved capacity benefits of a Li/SVO cell, alithium-SVO/CF_(x) mixture cell, or a Li-SVO/CF_(x) sandwich cathodecell, alternate electrolyte systems that are more conductive and morestable toward SVO than the binary solvent system of PC:DME have beeninvestigated.

One alternate electrolyte system is described in U.S. Pat. No. 5,753,389to Gan et al., which is assigned to the assignee of the presentinvention and incorporated herein by reference. The '389 patent to Ganet al. relates to an electrolyte system comprising 1.0M LiAsF₆/PC:DMEwith dibenzyl carbonate (DBC) used as an additive. In particular, theconductivity of a reference electrolyte and a test electrolytecontaining dibenzyl carbonate (DBC) as an organic carbonate additive,both electrolytes activating a Li/SVO cell, was investigated. Thereference electrolyte consisted of 1.0M LiAsF₆ dissolved in a 50:50, byvolume, mixture of PC:DME having a conductivity of 17.30 mohms/cm (priorart reference electrolyte No. 1). Dibenzyl carbonate (DBC) (0.01M) wasadded to prior art reference electrolyte No. 1 to provide prior art testelectrolyte No. 2.

The competing Li/SVO cells in the '389 patent to Gan et al., designatedas prior art Li/SVO cell No. 1 and prior art Li/SVO cell No. 2,respectively, were discharged under a 1K ohm load for 1 hour and thendischarged under a 2K ohm load. Once every two days these cells receiveda pulse train consisting of four 17.7 mA/cm², 10 second pulses with 15seconds rest between each pulse. Four pulse trains were applied to eachcell. In general, voltage delay in the first pulse can be observed forboth cells in the third and fourth pulse trains.

As shown in FIG. 2 , which is a reproduction of FIG. 1 from the '389patent, voltage delay is defined as pulse 1 end potential minus pulse 1minimum potential wherein in the pulse discharge curve 10, the pulse 1end potential is indicated at 12 and pulse 1 minimum potential isindicated at 14. A positive value indicates the existence of voltagedelay. The larger the positive value, the larger the voltage delay. Thebeneficial effects of the DBC organic additive on voltage delay are thusobtained by comparing the results of prior art test cell No. 2 havingthe DBC organic additive dissolved in the electrolyte solution withrespect to the prior art reference cell No. 1 at pulse trains 3 and 4.The results from pulse train 4 are summarized in Table 1 below.

TABLE 1 Effect of DBC Additive on Test Cell Performance: Voltage Delayin Fourth Pulse Train (V) Without DBC Additive With DBC Additive(P_(1end) − P_(1min)) (P_(1end) − P_(1min)) 0.13 0.07

The '389 patent to Gan et al. theorizes that the DBC organic carbonateadditive is reduced to form a product which deposits on the anodesurface. This surface film is ionically more conductive than the filmformed in the absence of the DBC additive and is responsible forincreased cell performance in terms of decreased voltage delay duringhigh current pulse discharge. Since lithium carbonate is known to form agood ionically conductive film on lithium surfaces, the reductivecleavage of the O—X and/or the O—Y bonds in the DBC carbonate additivemay produce lithium carbonate as the final product.

Thus, Li/SVO cells activated with an electrolyte comprising the PC andDME couple with a DBC additive exhibit good chemical and electrochemicalstability in comparison to electrolytes comprising the binary solventsystem of PC:DME, but that are devoid of the DBC additive. However, eventhough an electrolyte comprising PC and DME with a DBC additive isadvantageous in terms of its long-term performance and stability inLi/SVO, Li-SVO/CF_(x) mixture cells, and Li-SVO/CF_(x) sandwich cathodeprimary electrochemical cells, that electrolyte system still exhibits adegree of voltage delay that can be improved. Thus, a new electrolytesystem is needed that is chemically and electrochemically stable whilehaving a higher conductivity than the conventional solvent systemsdescribed above.

Accordingly, the present invention is directed to an electrolyte systemthat is more conductive than the conventional or prior art binary andternary solvent system electrolytes while being chemically andelectrochemically stable toward Li/SVO cells, Li-SVO/CF_(x) mixturecells, and Li-SVO/CF_(x) sandwich cathode primary electrochemical cells.

SUMMARY OF THE INVENTION

The present invention relates to a primary lithium electrochemical cellthat is suitable as a power source for an implantable device such as anICD or CRT-D device. The electrochemical cell contains a lithium orlithium alloy anode, a cathode containing silver vanadium oxide (SVO) ormanganese dioxide (MnO₂) alone, or with fluorinated carbon (CF_(x)) as amixture, or in discreet layers contacted to the opposite sides of acurrent collector in a configuration of SVO/current collector/CF_(x) orMnO₂/current collector/CF_(x). The cathode active materials arepreferably mixed with various conductive agents and binders. A porouspolyolefin or glass fiber separator is positioned between the anode andcathode to permit ionic conductivity between the opposite polarityelectrodes, but to prevent them from directly contacting each other. Thepreferred activating electrolyte consists of lithium hexafluoroarsenate(LiAsF₆) dissolved in a solvent system of from about 30:70 to about50:50, by volume, propylene carbonate (PC) and dimethoxyethane (DME) towhich lithium bis(oxalato)borate (LiBOB) and fluoroethylene carbonate(FEC) are added to thereby provide the highest energy and power density.More preferably, lithium bis(oxalato)borate (LiBOB) is added to theelectrolyte in an amount of about 0.005% wt. % to about 5% wt. %, andfluoroethylene carbonate (FEC) is added to the electrolyte in an amountranging from about 0.01 wt. % to about 10 wt. %.

These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description and to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph of the relative conductivity of 1.0M LiAsF₆ dissolvedin a binary mixture of PC:DME at various percentages of DME at 37° C.

FIG. 2 is a reproduction of the graph of FIG. 1 from U.S. Pat. No.5,753,389 to Gan et al. showing the pulse discharge curve 10 of acontrol electrochemical cell activated with an electrolyte comprising1.0M LiAsF₆ dissolved in a 50:50 mixture, by volume, of an aproticorganic solvent system of PC:DME according to conventional practice.

FIG. 3 is a graph comparing the pulse 1 minimum voltage of a pulsedischarged Li/SVO-CF_(x) sandwich cathode electrochemical cell activatedwith an electrolyte having 1.2M LiAsF₆ dissolved in a 30:70 mixture, byvolume, of PC/DME including 0.05M DBC according to convention practicein comparison to a similarly built cell activated with an electrolytehaving 1.2M LiAsF₆ dissolved in a 30:70 mixture, by volume, of PC/DMEbut without DBC and further including 0.1 wt. % LiBOB and 0.5 wt. % FECaccording to the present invention.

FIG. 4 is a graph comparing the pulse 1 last voltage of the prior artand present invention cells used to construct the graph shown in FIG. 3.

FIG. 5 is a graph comparing the pulse waveforms at about 30% depth ofdischarge for the prior art and present invention cells used toconstruct FIGS. 3 and 4 .

FIG. 6 is a graph comparing the pulse waveforms at about 54% depth ofdischarge for the prior art and present invention cells used toconstruct FIGS. 3 and 4 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term “pulse” means a short burst of electricalcurrent of a significantly greater amplitude than that of a pre-pulsecurrent, immediately prior to the pulse. A pulse train consists of atleast two pulses of electrical current delivered in relatively shortsuccession with or without open circuit rest between the pulses. Anexemplary pulse train consists of four 10 second pulses (40.0 mA/cm²)with a 15 second rest between each pulse.

An electrochemical cell according to the present invention is preferablyof a primary chemistry. For the cell to possess sufficient energydensity and discharge capacity required to meet the rigorousrequirements of implantable medical devices, the anode for the primarycell is a thin metal sheet or foil of lithium, pressed or rolled on ametallic anode current collector, i.e., preferably comprising titanium,titanium alloy or nickel. An alternate anode comprises a lithium alloyfor example, Li—Si, Li—Al, Li—B, Li—Mg and Li—Si—B alloys andintermetallic compounds. The greater the amounts of the secondarymaterial present by weight in the alloy, however, the lower the energydensity of the cell.

The anode current collector has an extended tab or lead contacted by aweld to a cell case of conductive metal in a case-negative electricalconfiguration. Alternatively, the anode may be formed in some othergeometry, such as a bobbin shape, cylinder or pellet, to allow for a lowsurface cell design.

It is generally recognized that for lithium cells, silver vanadium oxide(SVO) and, in particular, ϵ-phase silver vanadium oxide (AgV₂O_(5.5)),is preferred as the cathode active material for high rate cell dischargeapplications. This active material has a theoretical volumetric capacityof 1.37 Ah/ml. By comparison, the theoretical volumetric capacity ofCF_(x) (x=1.1) is 2.42 Ah/ml, which is 1.77 times that of ϵ-phase silvervanadium oxide. However, for powering a cardiac defibrillator, SVO ispreferred because it can deliver high current pulses or high energywithin a relatively short period of time. Although CF_(x) has highervolumetric capacity, it is not useful as the sole cathode activematerial in medical devices requiring a high rate discharge application.This is due to its relatively low to medium rate of dischargecapability. That is one of the reasons the lithium-SVO/CF_(x) sandwichcathode cells disclosed by Gan in U.S. Pat. No. 6,551,747 areadvantageous for providing both high energy capacity and high dischargerate in a single device.

The '747 patent to Gan, which is assigned to the assignee of the presentinvention and incorporated herein by reference, describes a sandwichcathode design preferably having fluorinated carbon (CF_(x))_(n) as afirst cathode active material of a relatively higher energy density, butof a relatively lower rate capability sandwiched between two currentcollectors, and preferably having silver vanadium oxide (SVO) as asecond cathode active material of a relatively lower energy density, butof a relatively higher rate capability in contact with the oppositesides of the current collectors.

Fluorinated carbon as the cathode active material of a relatively higherenergy density, but of a relatively lower rate capability is representedby the formula (CF_(x))_(n), wherein x ranges from about 0.1 to about1.9, and preferably from about 0.5 to about 1.2. An alternatefluorinated carbon material that is useful with the present invention is(C₂F)_(n), wherein n refers to the number of monomer units which canvary widely.

In addition to ϵ-phase silver vanadium oxide having x=1.0 and y=5.5 inthe general formula SM_(x)V₂O_(y), β-phase silver vanadium oxide havingin the general formula x=0.35 and y=5.8, and γ-phase silver vanadiumoxide having in the general formula x=0.80 and y=5.40 are also usefulcathode active materials of relatively low energy density, butrelatively high rate capability. For a more detailed description of suchcathode active materials, reference is made to U.S. Pat. No. 4,310,609to Liang et al. This patent is assigned to the assignee of the presentinvention and incorporated herein by reference.

Silver vanadium oxide (SVO) as the preferred first cathode activematerial having a greater rate capability than the second cathode activematerial is typically provided in a formulation or mixture facing theanode of, by weight, about 90% to about 95% SVO, about 1% to about 4%binder and about 1% to about 4% conductive diluent. Fluorinated carbon(CF_(x)) as the second active material is in contact with the other sideof the current collector and is preferably provided in a second activemixture having CF_(x) ranging from about 90% to 95% with the remainingconstituents being about 1% to about 5% of a binder and about 1% toabout 5% of one or more conductive diluents.

A suitable binder material is preferably a thermoplastic polymericmaterial. The term thermoplastic polymeric material is used in itsbroadest sense and any polymeric material which is inert in the cell andwhich passes through a thermoplastic state, whether it finally sets orcures, is included within the term “thermoplastic polymer”.Representative binder materials include polyethylene, polypropylene,polyimide, and fluoropolymers such as fluorinated ethylene, fluorinatedpropylene, polyvinylidene fluoride (PVDF), and polytetrafluoroethylene(PTFE) at about 1 to about 5 weight percent of the cathode mixture.Natural rubbers are also useful as the binder material with the presentinvention.

Up to about 10 weight percent of a conductive diluent is preferablyadded to the cathode mixture. Suitable conductive diluents includeacetylene black, carbon black and/or graphite. Metals such as nickel,aluminum, titanium and stainless steel in powder form are also useful asconductive diluents. A preferred conductive diluent for both SVO andCF_(x), is carbon. The use of carbon increases the conductivity of SVOand CF_(x). It is believed that this in turn improves the pulsedischarge performance of the cell, especially during the first third ofthe cell's discharge life.

Thus, an exemplary silver vanadium oxide formulation has about 94% SVOmixed with about 3% PTFE binder and about 3% carbonaceous diluent. Anexemplary fluorinated carbon formulation has about 91% CF_(x) mixed withabout 5% PTFE binder and about 4% carbonaceous diluent.

In addition to silver vanadium oxide, the first cathode active materialmay be comprised of V₂O₅, MnO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, TiS₂, Cu₂S,FeS, FeS₂, copper oxide, copper vanadium oxide, copper silver vanadiumoxide, and mixtures thereof. In addition to fluorinated carbon, thesecond cathode active material may be comprised of Ag₂O, Ag₂O₂, CuF₂,Ag₂CrO₄, MnO₂, and mixtures thereof. In a broader sense, however, it iscontemplated by the present invention that the first cathode activematerial may be any material which has a relatively higher ratecapability than the second cathode active material.

Another embodiment for a cathode according to the present invention hasthe first and second cathode active materials provided in a homogeneousmixture contacted to at least one side, and preferably both sides, of acurrent collector.

The cathode for an electrochemical cell according to the presentinvention having the previously described first and second cathodeactive materials may be made in a variety of “sandwich” configurations.The cathode current collector is preferably formed as an elongated sheethaving a first side and a second side with the disparate cathode activematerials contacted to the opposite sides thereof. The cathode currentcollector may be formed from stainless steel, titanium, tantalum,platinum, gold, aluminum, aluminum alloys, cobalt nickel alloys,nickel-containing alloys, highly alloyed ferritic stainless steelcontaining molybdenum and chromium, and nickel-, chromium-, andmolybdenum-containing alloys. In a preferred embodiment, the cathodecurrent collector is aluminum. In another embodiment, the currentcollector is titanium having a coating selected from the groupconsisting of graphite/carbon material, iridium, iridium oxide andplatinum provided thereon.

Additionally, the cathode may include two or more current collectorswith the overall design being independent of the exact screen or cellstack configuration. Exemplary cathode designs include:

SVO/current collector/CF_(x), with the SVO facing the anode;

CF_(x)/first current collector/SVO, with the CF_(x) facing the anode;

SVO/CF_(x)/current collector/CF_(x)/SVO;

SVO/first current collector/CF_(x)/second current collector/SVO;

SVO/first current collector/SVO/CF_(x)/SVO/second current collector/SVO;

CF_(x)/first current collector/SVO/second current collector/CF_(x); and

CF_(x)/first current collector/CF_(x)/SVO/CF_(x)/second currentcollector/CF_(x).

In embodiments of the present electrochemical cells in which thedisparate cathode active materials are delivered in the form of a pasteor slurry applied to opposite sides of a current collector, the slurriesare provided by dissolving or dispersing the desired cathode activematerial, conductive diluent and binder in a solvent. Suitable solventsinclude water, methyl ethyl ketone, cyclohexanone, isophorone,N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamideN,N-dimethylacetamide, toluene, and mixtures thereof.

To prevent internal short circuit conditions, the sandwich cathode ofthe present electrochemical cells is separated from the lithium anode bya suitable separator material. The separator is of an electricallyinsulative material, and the separator material is also chemicallyunreactive with the anode and cathode active materials and bothchemically unreactive with and insoluble in the electrolyte. Inaddition, the separator material has a degree of porosity sufficient toallow flow there through of the electrolyte during electrochemicalreactions of the cell.

Illustrative separator materials include fabrics woven fromfluoropolymeric fibers including polyvinylidine fluoride,polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethyleneused either alone or laminated with a fluoropolymeric macroporous film,non-woven glass, polypropylene, polyethylene, glass fiber materials,ceramics, a polytetrafluoroethylene membrane commercially availableunder the designation ZITEX (Chemplast Inc.), a polypropylene membranecommercially available under the designation CELGARD (Celanese PlasticCompany, Inc.), and a membrane commercially available under thedesignation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.).

The electrochemical cells of the present invention further include anonaqueous, ionically conductive electrolyte which serves as a mediumfor migration of ions between the anode and the cathode duringelectrochemical reactions of the cell. The electrochemical reaction atthe electrodes involves conversion of ions in atomic or molecular formswhich migrate from the anode to the cathode. Thus, suitable nonaqueouselectrolytes are substantially inert to the anode and cathode activematerials, and they exhibit those physical properties necessary forionic transport, namely, low viscosity, low surface tension andwettability.

A suitable electrolyte has an inorganic, ionically conductive lithiumsalt dissolved in a mixture of aprotic organic solvents comprising a lowviscosity solvent and a high permittivity solvent. The inorganic,ionically conductive salt serves as the vehicle for migration of thelithium anode ions to intercalate or react with the cathode activematerials. Known lithium salts that are useful as a vehicle fortransport of lithium ions from the anode to the cathode include LiPF₆,LiBF₄, LiAsF₆, LiSbF₆, LiClO₄, LiO₂, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃, LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₃, LiC₆FSO₃, LiO₂CCF₃ , LiSO₆ F, LiB(C₆H₅)₄,LiCF₃SO₃, and mixtures thereof.

Low viscosity solvents useful with the present invention include esters,linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran(THF), methyl acetate (MA), diglyme, trigylme, tetragylme, dimethylcarbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE),1-ethoxy, 2-methoxyethane (EME), ethyl methyl carbonate, methyl propylcarbonate, ethyl propyl carbonate, diethyl carbonate, dipropylcarbonate, and mixtures thereof. High permittivity solvents includecyclic carbonates, cyclic esters and cyclic amides such as propylenecarbonate (PC), ethylene carbonate (EC), butylene carbonate,acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethylacetamide, γ-valerolactone, γ-butyrolactone (GBL),N-methyl-pyrrolidinone (NMP), and mixtures thereof. In theelectrochemical cells of the present invention, the preferredelectrolyte comprises 0.8M to 1.5M LiAsF₆ or LiPF₆ dissolved in a 20:80to 50:50 mixture, by volume, of propylene carbonate (PC) as thepreferred high permittivity solvent and 1,2-dimethoxyethane (DME) as thepreferred low viscosity solvent, more preferably, 1.2M LiAsF₆ dissolvedin a 30:70 mixture, by volume, of PC and DME.

To this electrolyte, lithium bis(oxalato)borate (LiBOB) andfluoroethylene carbonate (FEC) are added to provide improved energy andpower density. More preferably, lithium bis(oxalato)borate (LiBOB) isadded to the electrolyte in an amount of about 0.005% wt. % to about 5%wt. %, most preferably in an amount of about 0.1 wt. %, andfluoroethylene carbonate (FEC) is added to the electrolyte in an amountranging from about 0.01 wt. % to about 10 wt. %, and most preferably inan amount of about 0.5 wt. %. Another preferred electrolyte according tothe present invention consists of 1.2M LiAsF₆ dissolved in a 50:50mixture, by volume, of PC:DME with 0.1 wt. % LiBOB and 0.5 wt. % FECadditives.

The assembly of the cells described herein is preferably in the form ofa prismatic configuration. The cells may also be in the form of a woundelement configuration. That is, the fabricated anode, cathode andseparator may be wound together in a “jellyroll” type configuration or“wound element cell stack” such that the anode is on the outside of theroll to make electrical contact with the cell case in a case-negativeconfiguration. Using suitable top and bottom insulators, the wound cellstack is inserted into a metallic case of a suitable size dimension. Themetallic case may comprise materials such as stainless steel, mildsteel, nickel-plated mild steel, titanium, tantalum, aluminum, andniobium, but not limited thereto, so long as the metallic material iscompatible for use with the other cell components.

The cell header may be comprised of a metallic disc-shaped body with afirst hole to accommodate a glass-to-metal seal/terminal pin feedthroughand a second hole for electrolyte filling. The glass used is of acorrosion resistant type having up to about 50% by weight silicon suchas CABAL 12, TA 23, FUSITE 425 or FUSITE 435. The positive terminal pinfor the feedthrough preferably comprises titanium although molybdenum,aluminum, nickel alloy, niobium, and stainless steel can also be used.The cell header is typically of a material similar to that of the case.The positive terminal pin supported in the glass-to-metal seal is, inturn, supported by the header, which is welded to the case containingthe electrode stack. The cell is thereafter filled with the electrolytesolution described hereinabove and hermetically sealed such as byclose-welding a stainless-steel ball over the fill hole, but not limitedthereto.

The above assembly describes a case-negative cell, which is thepreferred construction of the cells of the present invention. As is wellknown to those skilled in the art, however, the present electrochemicalcells can also be constructed in a case-positive configuration.

The following example describes the manner and process ofelectrochemical cells according to the present invention. The examplesets forth the best mode contemplated by the inventors of carrying outthe invention, but it is not to be construed as limiting.

EXAMPLE

Ten cells were built containing anodes of lithium metal pressed onto anickel current collector ribbon. Cathodes were built having cathodeactive materials pressed onto two layers of aluminum foil currentcollector to provide an electrode configuration of: SVO/currentcollector/CF_(x)/current collector/SVO. The SVO material was ϵ-phase SVOwith a stoichiometry formula of Ag₂V₄O₁₁. A prismatic cell stackassembly configuration with two layers of microporous membrane(polyethylene) separator sandwiched between the anode and cathode wasprepared. The electrode assembly was then hermetically sealed in astainless-steel casing in a case negative configuration.

Prior art cells 1 to 5 were activated with an electrolyte of 1.2M LiAsF₆dissolved in a solvent system of 0.05M DBC/PC: DME=30:70, by volume.Present invention cells 6 to 10 were activated with an electrolyte of1.2M LiAsF₆ dissolved in a solvent system of PC: DME=30:70, by volume(no DBC) and an additional 0.1 wt. % of lithium bis(oxalato)borate(LiBOB) and 0.5 wt. % of fluoroethylene carbonate (FEC). The theoreticalcapacity of the ten cells was 1.883 Ah.

The cells were discharged at 72° C. under a 4.32 kohm background load.The pulse regimen consisted of four consecutive 10-second pulses at3,228 mA with 15 seconds on background load applied once every 7 days.

FIGS. 3 and 4 are graphs showing the same background voltages for thefive present invention cells 6 to 10 (generally indicated as curve 30).The background voltages for the cells generally overlay each other.

Curves 32 and 34 in FIG. 3 generally indicate the respective pulse 1minimum voltages of the five prior art cells 1 to 5 in comparison tocells 6 to 10 of the present invention. Curves 42 and 44 in FIG. 4generally indicate the respective pulse 1 last voltages of the fiveprior art cells 1 to 5 in comparison to cells 6 to 10 of the presentinvention. It is noted that the pulse 1 minimum voltages and the pulse 1last voltages for all the cells are very similar in FIGS. 3 and 4 . Thisindicates that neither the prior art cells 1 to 5 nor the presentinvention cells 6 to 10 exhibited voltage delay. However, the pulse 1minimum voltages and the pulse 1 last voltages for the present inventioncells 6 to 10 are significantly higher than those of the prior art cells1 to 5, indicating that the removal of DBC and the addition of LiBOB andFEC has a significant effect on the improved discharge performance ofthe present invention cells 6 to 10.

FIGS. 5 and 6 are graphs comparing the pulse waveforms at about 30% and54% depth of discharge (DoD), respectively, for a representative one ofthe prior art cells 1 to 5 in comparison to a representative one of thepresent invention cells 6 to 10. In particular, curves 52 and 54 in FIG.5 are respective representative waveforms for one of the prior art cells1 to 5 in comparison to one of the present invention cells 6 to 10 at30% DoD. Throughout the pulse discharge waveform, the representativepresent invention cell maintained a higher voltage than therepresentative prior art cell. Curves 62 and 64 in FIG. 6 are respectiverepresentative waveforms for one of the prior art cells 1 to 5 incomparison to one of the present invention cells 6 to 10 at 54% DoD.Throughout the pulse discharge waveform, the representative presentinvention cell maintained a higher voltage than the representative priorart cell.

As discussed in the Prior Art section of this specification above,voltage delay is present when the pulse 1 end potential minus the pulse1 minimum potential is a positive value (FIG. 2 ). However, in FIGS. 5and 6 , voltage delay is not present as the pulse minimum potentials andthe pulse end potentials for pulses 1 to 4 coincide.

Thus, the addition of LiBOB and FEC as additives to an electrolytecomprising a lithium salt dissolved in a solvent system of a lowviscosity solvent and a high permittivity solvent, but without dibenzylcarbonate (DBC), improves the performance of an implantable lithiumelectrochemical cell activated with the electrolyte. The addition ofLiBOB and FEC to such electrolytes also increases both the volumetricenergy and power density of an implantable lithium electrochemical cell,and significantly improves the performance of an implantable medicaldevice powered by such cells, all to the benefit of the patient.

It is appreciated that various modifications to the inventive conceptsdescribed herein may be apparent to those of ordinary skill in the artwithout departing from the spirit and scope of the present invention asdefined by the appended claims.

What is claimed is:
 1. An electrochemical cell, comprising: a) a casing;b) an electrode assembly housed inside the casing, the electrodeassembly comprising: i) a lithium anode; ii) a cathode comprising silvervanadium oxide (SVO); and iii) a separator residing between the anodeand the cathode to prevent them from direct physical contact with eachother; and c) a nonaqueous electrolyte provided in the casing inphysical contact with the anode and the cathode, the electrolytecomprising: i) LiAsF₆ in a molar ratio ranging from about 0.8M to about1.5M dissolved in a solvent system comprising propylene carbonate (PC)and dimethoxy ethane (DME) in a ratio ranging from about 20:80 PC:DME toabout 50:50 PC:DME, by volume; and ii) an additive mixture dissolved inthe solvent system, the additive mixture comprising: A) lithiumbis(oxalato)borate (LiBOB) in an amount ranging from about 0.05 wt. % toabout 1 wt. %; and B) fluoroethylene carbonate (FEC) in an amountranging from about 0.1 wt. % to about 1.0 wt. %.
 2. The electrochemicalcell of claim 1, wherein the cathode further comprises fluorinatedcarbon.
 3. The electrochemical cell of claim 2, wherein the fluorinatedcarbon is (C₂F)_(n) or the fluorinated carbon has the formula(CF_(x))_(n) with x ranging from about 0.1 to about 1.9, and mixturesthereof.
 4. The electrochemical cell of claim 2, wherein the silvervanadium oxide is mixed with the fluorinated carbon.
 5. Theelectrochemical cell of claim 2, wherein the silver vanadium oxide (SVO)and the fluorinated carbon (CF_(x)) are contacted to opposite sides ofat least one current collector in a configuration selected from: a)SVO/current collector/CF_(x), with the SVO facing the anode; b)CF_(x)/first current collector/SVO, with the CF_(x) facing the anode; c)SVO/CF_(x)/current collector/CF_(x)/SVO; d) SVO/first currentcollector/CF_(x)/second current collector/SVO; e) SVO/first currentcollector/SVO/CF_(x)/SVO/second current collector/SVO; f) CF_(x)/firstcurrent collector/SVO/second current collector/CF_(x); and g)CF_(x)/first current collector/CF_(x)/SVO/CF_(x)/second currentcollector/CF_(x).
 6. An electrochemical cell, comprising: a) a casing;b) an electrode assembly housed inside the casing, the electrodeassembly comprising: i) a lithium anode; ii) a cathode comprising silvervanadium oxide (SVO) and fluorinated carbon; and iii) a separatorresiding between the anode and the cathode to prevent them from directphysical contact with each other; and c) a liquid, nonaqueouselectrolyte provided in the casing in physical contact with the anodeand the cathode, the electrolyte comprising: i) LiAsF₆ in a molar ratioranging from about 0.8M to about 1.5M dissolved in a solvent systemconsisting of propylene carbonate (PC) and 1,2-dimethoxyethane (DME) ina ratio ranging from about 20:80 PC:DME to about 50:50 PC:DME, byvolume; and ii) an additive mixture consisting of: A) lithiumbis(oxalato)borate (LiBOB) in an amount ranging from about 0.05 wt. % toabout 1.0 wt. %; and B) fluoroethylene carbonate (FEC) in an amountranging from about 0.1 wt. % to about 1.0 wt. %.
 7. The electrochemicalcell of claim 6, wherein the fluorinated carbon is (C₂F)_(n) or thefluorinated carbon has the formula (CF_(x))_(n) with x ranging fromabout 0.1 to about 1.9, and mixtures thereof.
 8. The electrochemicalcell of claim 6, wherein the silver vanadium oxide is mixed with thefluorinated carbon.
 9. The electrochemical cell of claim 6, wherein thesilver vanadium oxide (SVO) and the fluorinated carbon (CF_(x)) arecontacted to opposite sides of at least one current collector in aconfiguration selected from: a) SVO/current collector/CF_(x), with theSVO facing the anode; b) CF_(x)/first current collector/SVO, with theCF_(x) facing the anode; c) SVO/CF_(x)/current collector/CF_(x)/SVO; d)SVO/first current collector/CF_(x)/second current collector/SVO; e)SVO/first current collector/SVO/CF_(x)/SVO/second current collector/SVO;f) CF_(x)/first current collector/SVO/second current collector/CF_(x);and g) CF_(x)/first current collector/CF_(x)/SVO/CF_(x)/second currentcollector/CF_(x).